The invention relates to an electrochemical cell comprising a negative electrode whose electrochemically active material consists of an intermetallic compound which forms a hydride with hydrogen and which has the CaCu.sub.5 -structure and a catalytic material at the surface of which hydrogen exhibits a large electrochemical activity.
The invention also relates to an electrochemically active material for use in such a cell and to a method of manufacturing such a material.
The electrochemical cell may be in open communication with the atmosphere or may be sealed from the atmosphere. A cell which is sealed from the atmosphere may comprise a valve which is dimensioned such that it becomes operative at a predetermined pressure.
In a rechargeable cell of the sealed type, the electrochemically active part of the positive electrode consists of, for example, nickel hydroxide, silver oxide or manganese oxide, nickel hydroxide being generally preferred for practical reasons.
An electrolyte is used in the cell, which generally consists of an aqueous solution of one or more alkali hydroxides such as lithium hydroxide, sodium hydroxide and potassium hydroxide, having a pH in excess of 7.
The cell may further comprise a separator which separates the electrodes electrically, but permits transport of ions and gas. The separator may consist of synthetic resin fibres (woven or non woven), for example, of polyamide fibres or polypropylene fibres.
Such an electrochemical cell is described in European Patent Application EP-A1-0251384 (U.S. Pat. No. 4,699,856). In this application, one or more of the metals Pd, Pt, Ir and Rh are added to the electrochemically active material to improve the loadability at low temperature and the activation speed of the cell.
A disadvantage of the known cell is that it requires the use of a relatively large quantity of a noble metal. When the noble metal is used in the form of a monolayer on the grains of the hydride-forming material, as is also described in EP-A1-0251384, a smaller quantity of the noble metal is required, but in this case, however, a labor-intensive electrochemical treatment is necessary to apply the monolayer. Moreover, for some applications it is desirable to further increase the loadability of the cell, in particular at low temperatures.